At first glance, Ubiquity strikes me as Quicksilver for the Web. I suspect it will be much more.

Install a single plugin, then summon Ubiquity’s command box with a keysequence. You can also use a contextual menu (right). Pre-installed commands are sorted in order of expected use. ‘Tag’ uses Firefox’s local tagging capability. To add delicious bookmarks, you could try delicious plugin code from Ryan Sonnek.

Aza Raskin’s Ubiquity Intro Video gives a sense of the current capabilities. For instance, Example #2 shows that highlighting Craigslist apartment listings and invoking ‘map these’ generates a map of the listings.

Step 1: Highlight Craigslist apartment listings

Step 2: Invoke Ubiquity's 'map-these' command

Step 3: Voila! Mapped listings. (Less clicking, more mapping!)

Nice, eh? Go try Ubiquity. Or, if you want to read more first, there are plenty of options: